Active pixel sensor arrays contain an array of individual photo sensors that collectively capture digital image data. These sensor arrays, which are commonly referred to as complementary metal oxide semiconductor (CMOS) image sensors since they are commonly fabricated using CMOS processing technology, are used in a wide variety of commonplace consumer electronic devices, such as digital still cameras, digital video cameras, and image copying devices. FIG. 1 is a functional block diagram of a conventional imaging device 100 that includes an active pixel sensor array 102 formed by a plurality of photo sensors or pixels PS arranged in rows and columns. Each pixel PS in a given row is coupled to an associated row line R1-RN and each pixel in a given column is coupled to an associated column line C1-CM.
A row decoder and control circuit 104 provides row activation signals on the row lines R1-RN to sequentially activate a selected row of pixels PS. Column amplifiers 106 develop a differential voltage DV for each activated pixel PS where the differential voltage corresponds to the difference between the voltage on the column line C coupled to that pixel and the voltage on a reference column line CR, which corresponds to the Mth column in the array 102 so this column line is alternatively indicated in parentheses as column line CM. The last column of pixels PSR1M-PSRNM is a reference column coupled to reference column line CR, with all these references pixels being suitably covered so that no light from an image being captured is incident upon these reference pixels.
In operation, to capture an image the row decoder and control circuit 102 initially supplies reset signals on the row lines R to reset all the pixels PS. Each pixel PS includes a photo-detector (not shown) that is typically a photo diode PD, and when reset these photo detectors are charged to a maximum voltage. The pixels PS in the array 102 are then exposed to incident light from the image being captured and this incident light discharges the photo detectors of each pixel PS by an amount that is a function of the intensity of the incident light. As a result of this discharge, after the exposure time each pixel PS has a voltage on the corresponding photo detector PS having a value that is a function of the intensity of the incident light.
At this point, the row decoder and control circuit 104 sequentially activates each row of pixels PS, causing each activated pixel to develop a voltage on the corresponding column line C indicating the voltage at the photo detector PS of that pixel. The column amplifiers 106 then amply this voltage on each column line C and output this amplified voltage as a pixel voltage PVNM for each pixel in the activated row. In this way, a pixel voltage PVNM for each pixel in the array 102 is read out of the array 102. As will be understood by those skilled in the art, to reduce noise and improve the accuracy of the pixel voltages PV read from conventional arrays 102, a differential approach may be utilized where each pixel voltage is a differential voltage defined by the voltage on a given column line C and a voltage on a reference column line coupled to reference pixels contained in each row of the array, as is described in more detail in commonly owned U.S. patent application Ser. No. 11/601,346 entitled DIM ROW SUPPRESSION SYSTEM AND METHOD FOR ACTIVE PIXEL SENSOR ARRAYS to Mentzer (“Mentzer”), which is incorporated herein by reference. For ease of description, the pixels voltage PV from the column amplifiers 106 are assumed in the following description to correspond simply to the voltages developed by activate pixels PS on the associated column lines C.
Regardless of how the exact way in which the pixel voltages PVNM are developed, these voltages are analog voltages and must then be digitized and stored to complete the capture of the image. To do this, an analog-to-digital (A/D) converter (not shown) converts each analog pixel voltage PV into a corresponding digital value having a number of bits. These digital values from all the pixels PX in the active pixel sensor array 102 collectively form a captured digital image file. This digital image file may then typically be viewed on the imaging device 100 containing the active pixel sensor array 102, such as where the device is a digital camera, or the files may be transferred to another device such as a printer and used to generate a hard copy of the captured image, or transferred to a computer or television for viewing on a suitable display.
As with most any type of electronic device, the active pixel sensor array 102 must be tested after manufacture of the array to verify proper operation. Typically, two types of errors are detected during testing of active pixel sensor arrays 102. The first type of error results from a pixel PS that is not operating properly, such as a “hot pixel” or a “stuck-at” pixel. A stuck-at pixel is a pixel PS that is stuck at a particular voltage, and thus regardless of incident light upon the photo detector of that pixel the pixel, when activated, provides the same “stuck-at” voltage value on the column line C. A hot pixel is a pixel PS having a high leakage current that causes the photo detector to discharge even when there is no light incident upon the photo detector. Thus, if a pixel PS is a hot or stuck-at pixel, the pixel provides a voltage that does not accurately indicate the intensity of light incident upon that pixel.
The second type of error is generally referred to as “fixed-pattern” error. This type of error can be either a row-wise or column-wise error for an entire row or column of pixels in the image sensor array. One example of a fixed-pattern error is dim row error that results when the previously mentioned differential approach is used. When the reference pixel or pixels in a given row is a hot pixel, such a hot pixel will result in an erroneous voltage from the reference pixel, which ideally provides a maximum voltage equal to the reset voltage. This erroneous voltage on the reference column line will then be subtracted from the voltage on the column line C for every other pixel in the associated row. Since the voltage on the reference column line is less than the ideal maximum value, the differential voltage values from the associated entire row of pixels PS will be less than desired, resulting in the row being too dim relative to neighboring rows, as will be understood by those skilled in the art. Other errors could result in an entire row of pixels being too bright relative to neighboring rows, or an entire column of pixels being too dim or bright. Such bright rows or columns of pixels are very noticeable when viewing a captured digital image and are therefore undesirable.
In conventional testing of active pixel sensor arrays 102, an external tester (not shown) controls the array or the imaging device 100, such as a cell phone or digital camera, containing the array to test proper operation of the array. To test the active pixel sensor array 102, a test image (not shown) is positioned within a field of view of the active pixel sensor array. The pixels PS in the array 102 are reset, meaning each photo detector is charged, and the image captured by exposing the test image to the pixels in the array for an appropriate exposure time. Typically, the test image is a black image. Data captured by the active pixel sensor array 102 is then provided to the external tester which analyzes the data to detect erroneous pixels in the array, such as hot pixels or stuck at pixels as previously discussed. In addition, the tester analyzes data to detect fixed pattern errors such dim rows or columns or overly bright rows or columns.
External testers are slow in that the rate at which data can be transferred from the active pixel sensor array 102 to the tester and thereafter analyzed by the tester is sufficiently long that it takes a relatively long time to test each array or device containing that array. Ideally, the test time would be reduced to enable the active pixel sensor arrays and devices containing such arrays to be more quickly tested.